It is known to provide a vehicular exterior rearview mirror assembly that has a mirror casing and reflective element that are adjustable relative to the side of the vehicle to which the mirror assembly is mounted, with a heater pad disposed at the rear of the reflective element. Such automotive exterior rearview mirrors are often heated for the purpose of clearing frost and ice from the viewing surface. The wattage to do this in an acceptable amount of time may, in some situations, over heat the mirror glass substrate, which may cause cracking or failure of the glass substrate or substrates of the mirror reflective element.